A standard mixing valve (see U.S. Pat. Nos. 4,621,659 and 4,540,023) used in a single-lever faucet has a normally cylindrical housing centered on an axis and a valve plate closing the bottom of the housing and formed with a hot-water inlet port, a cold-water inlet port, and an outlet port. A control plate sits flat atop this valve plate and is formed with a cavity that normally overlies the outlet port and that can also be brought into registration over the inlet ports, either singly or jointly. A lever pivoted on the housing is connected to this control plate to slide it atop the valve plate, thereby determining the amount of overlap of the control-plate cavity with the inlets to determine the amounts of hot and cold water to be shunted through this cavity to the outlet port.
A problem with such a mixing valve, which is typically provided as a separate cartridge that is suitable for use as a replacement part when the original valve wears out, is that it can fall apart prior to mounting unless handled very carefully. The bottom plate, which can be separate from or the same as the valve plate, can fall out, and the seals around the ports on the bottom of this plate also can fall out. Furthermore, the assembly must be made to very close tolerances so that the control and valve plates stay in contact with each other.